1. Field of the Invention
The present invention is directed generally to a wafer carrier for use in transportation and storage of semiconductor wafers, and, more particularly, to a wafer carrier having both a rigid structure and resistance to corrosive environments.
2. Description of the Background
Wafer carriers, also known as "wafer boats", are well known in the art of semiconductor fabrication. Wafer carriers are used to store and transport semiconductor wafers during semiconductor fabrication. Wafer carriers are usually transported by robots, although they may also be transported manually. Wafer carriers are often subjected to high temperature environments, such as during diffusion and annealing steps. In addition, wafer carriers are exposed to corrosive environments, such as chemical baths used for etching and cleaning the wafers. Those chemical baths may include chemicals such as hydrofluoric acid, sulfuric acid, phosphoric acid, ammonium hydroxide, hydrochloric acid, hydrogen peroxide, isopropyl alcohol, and nitric acid. Throughout the high temperature and corrosive environments, a wafer carrier must remain structurally sound and chemically stable so as not to damage or contaminate the wafers which it is carrying.
It is known to make wafer carriers from rigid materials such as silicon carbide (SiC) and quartz, as disclosed in U.S. Pat. No. 5,443,649, issued to Sibley, and U.S. Pat. No. 5,468,112, issued to Ishii et al.
Although quartz is the most commonly used material for wafer carriers, it has many deficiencies, such as being hydrophilic, making it difficult to dry after a "water rinse" step, and potentially leaving water marks on the wafers. In addition, quartz is susceptible to etching by commonly used chemicals in the fabrication process.
SiC retains its structural rigidity at high temperatures, so it can be used in high temperature process steps. SiC wafer carriers, however, have several serious deficiencies. For example, like quartz, SiC is hydrophilic and may result in water spots on the wafers. In addition, SiC wafer carriers are typically cast to the desired form, which results in porous SiC. Unfortunately, porous SiC is unsuitable for use as a wafer carrier because it tends to collect contaminants and then disperse them, thereby contaminating the wafers. In addition, porous SiC will also trap water which, upon heating of the wafer carrier, may expand rapidly and damage the wafer carrier. As a result, the porous SiC carrier must be completely coated with a layer of non-porous SiC. All SiC is brittle and a coating of non-porous SiC will frequently crack or chip, exposing the porous SiC within. As an alternative, U.S. Pat. No. 5,443,649 discloses a method to create a wafer carrier composed entirely of non-porous SiC, thereby eliminating the risks associated with using porous SiC. That method, however, requires a complex and expensive process of creating and subsequently removing or destroying an underlying structure on which the non-porous SiC can be deposited by chemical vapor deposition (CVD). If that underlying structure is allowed to remain in the wafer carrier, it may contaminate wafers in the same manner as porous SiC.
SiC in general, including non-porous SiC, is inherently unsuitable for use as a wafer carrier. SiC is brittle, and when wafers are placed in a SiC wafer carrier the wafers rub against the brittle SiC wafer carrier, generating particulates which can contaminate the wafers within the wafer carrier.
An alternative to SiC wafer carriers are wafer carriers constructed from resin mold products, such as tetrafluoroethylene/perfluoroalkyl-vinylether copolymer (PFA) and polypropylene (PP). Those materials are resistant to the chemicals commonly used to clean and etch wafers. Unfortunately, wafer carriers made from those materials are flexible, and they tend to bend and sag after relatively short periods of use. Some wafer carriers currently hold one hundred or more wafers, and resin mold products are unsuitable for use in those large wafer carriers because resin mold products will flex and bend under their own weight. Furthermore, resin mold products become even softer and more flexible in high temperature process steps commonly used in the fabrication of semiconductors.
U.S. Pat. No. 5,468,112, issued to Ishii et al., (the "Ishii Patent") discloses a wafer carrier constructed of resin mold products such as PFA and PP, as well as a wafer carrier constructed of SiC. The Ishii Patent, however, does not disclose a resin mold product and SiC used together to form a wafer carrier. In fact, the Ishii Patent does not even address the problems of structural and chemical instability in a wafer carrier. The Ishii Patent focuses on designing a wafer carrier which supports the wafers in a level manner, and prevents the wafers from being tilted within the wafer carrier. The Ishii Patent also teaches a wafer processing system which is not dependant on the use of the wafer carrier disclosed in the Ishii Patent.
U.S. Pat. No. 5,443,649, issued to Sibley, (the "Sibley Patent") discloses a wafer carrier made from SiC, but does not disclose a wafer carrier made from both SiC and a resin mold product. The Sibley Patent focuses on providing a method for creating a wafer carrier constructed entirely from non-porous SiC formed by CVD. That method requires the use of a support structure, constructed of graphite, on which the SiC, can be deposited. Following the deposition of SiC the support structure must be removed so that it does not contaminate the wafers during processing steps. Although the Sibley Patent recognizes the deficiencies of quartz and porous SiC, it neither recognizes nor provides a solution for the general problems suffered by wafer carriers constructed from non-porous SiC. Instead, the Sibley Patent provides an improved carrier which is superior to the porous SiC carriers previously used, but one which is still subject to deficiencies of SiC.
It is known to construct a wafer carrier from several SiC rods within a PFA shell. The wafer carrier is constructed by first creating a wafer carrier constructed entirely from PFA, boring holes in the wafer carrier where the rods are to be located, and inserting straight rods into the holes within the wafer carrier. Once the rods are inserted in the wafer carrier, the holes may be plugged or filled in a known manner. That structure obviously limits the designer of a wafer carrier to use unconnected, straight rods. Typically, the rods are used to form a "backbone" near the wafer platforms and running the length of the wafer carrier. The rods are not connected, and so there is a gap between them. The PFA coating covers the rods and bridges the gaps between the rods to bind the rods together. The rods provide strength to the wafer carrier, and the PFA coating provides resistance to etching by chemicals commonly used in the semiconductor fabrication process. Those wafer carriers, however, still suffer from the inherent weakness of the PFA material. Although the SiC rods remain rigid, the wafer carrier tends to sag and bend at the gaps between the rods. The bending and sagging causes stress on the PFA structure, which weakens the PFA causing even more bending and sagging. As a result, the wafer carriers have a limited useful life before their lack of rigidity makes them unusable. Such wafer carriers have been known to exhibit failure rates of 50% in a one month period. The weakness of the PFA, and the resultant bending and sagging, also limits the size of wafer carrier which can be constructed. Furthermore, excessive bending and sagging will eventually cause the PFA to split, exposing the rods within the wafer carrier, leading to contamination of the chemical baths and the wafers carried within the wafer carrier.
Thus, the need exists to provide a wafer carrier which is both resistant to chemicals which are commonly used to clean and etch wafers, and structurally sound so that it is rigid enough to support the wafers and maintain the structural integrity of the wafer carrier.